The invention relates to a hitch for movably coupling an implement to a utility vehicle, and in particular, to a hitch having a pair of hydraulic cylinders for controlling the position of the implement with respect to the utility vehicle.
Implement hitches are known for coupling implements to agricultural and industrial vehicles. For example, a three point hitch may be attached to the rear of an agricultural tractor. Such a hitch normally includes two lower arms and an upper arm whose free ends are equipped with catch hooks or other fastening means for fastening attached implements. Usually each of the lower arms is connected through lift struts with a common lift shaft, that can be rotated hydraulically. This results in a rigid mechanical connection between the two lower arms and an enforced synchronization of the lower arms.
U.S. Pat. No. 6,253,859 shows a hitch, in which each lower draft link can be raised and lowered individually by a separate lift cylinder, instead of a lift strut and a lift shaft. In such a hitch, which is often used with front power lifts, the synchronization of the two lower arms must be guaranteed by an active control of the hydraulic cylinders. This requires additional power consumption and a corresponding control system.
An object of the present invention is to provide a hitch with separate lift cylinders which are synchronously operated without the application of additional energy sources and without additional control systems.
This and other objects are achieved by the present invention wherein an implement attachment interface or hitch couples an implement to a utility vehicle. The hitch includes a pair of hydraulic cylinders which operate to control the position of the implement. The hydraulic cylinders are communicated with a gear-type flow divider or motor/pump unit which synchronizes the operation of the cylinders without requiring an additional power source.
The motor/pump unit synchronizes the cylinders even during floating operation, such as when the hydraulic cylinders are unpressurized. Floating operation can be used during ground breaking or during plowing with a plow. Alternatively, the lift cylinders can be connected with a conventional depth control system to control implement depth.
The invention assures the synchronous operation of the hydraulic cylinders in the floating position without mechanical connections. No common lift shaft or other mechanical connecting means between the coupling elements is required. An additional hydraulic pump is not required during floating operation to maintain a constant side-to-side orientation of the implement. Defects that could occur from an active control of the lift cylinders, can be avoided.
It is possible to connect only the piston end chambers or only the rod end chambers of the two hydraulic cylinders with the motor/pump unit. However, preferably, the hydraulic cylinders are double acting hydraulic cylinders and the two piston end chambers and the rod end chambers are connected to the rotors of the motor/pump unit.
Preferably, the motor/pump unit includes at least two rotors arranged on a common shaft. The rotors are pairs of gears that are configured as pump/motor units. Each rotor is connected on the one hand with the piston end chamber of one of the two hydraulic cylinders and on the other hand with a reservoir, such as the fluid reservoir for the operating hydraulic fluid of the utility vehicle.
Preferably, the motor/pump unit includes at least two rotors arranged on a common shaft, each of which is connected on the one hand with the rod end chamber of one of the two hydraulic cylinders and on the other hand with a reservoir.
Alternatively, the motor/pump unit includes four rotors arranged on a common shaft. Two of these rotors have a larger intake capacity and are connected with the piston end chamber of one of the two hydraulic cylinders. Two of the rotors have a smaller intake capacity and are connected with the rod end chamber of one of the two hydraulic cylinders. All rotors are connected on the other hand with a reservoir.
In a further preferred embodiment, the motor/pump unit includes at least two rotors arranged on a common shaft of which a first rotor is connected with the piston end chamber of a first hydraulic cylinder and a second rotor is connected with the rod end chamber of a second hydraulic cylinder and both rotors, on the other hand, are connected with a reservoir.
Two motor/pump units may be provided where a first motor/pump unit includes two rotors arranged on a common first shaft, of which a first rotor is connected with the piston end chamber of a first hydraulic cylinder and a second rotor is connected with the rod end chamber of a second hydraulic cylinder. A second motor/pump unit also includes two rotors arranged on a second common shaft of which a first rotor is connected with the rod end chamber of the first hydraulic cylinder and a second rotor is connected with the piston end chamber of the second hydraulic cylinder. All rotors are connected, on the other hand, with a reservoir.
Preferably, the intake capacity of the rotors conforms with each of the associated hydraulic cylinder chambers. If, for example, the two piston chambers of a double acting hydraulic cylinder are connected with a rotor of a motor/pump unit that include two rotors, then the synchronous operation of the hydraulic cylinders is guaranteed when the intake capacity of the first rotor is proportional to the effective piston surface of the associated first cylinder volume, and simultaneously the intake capacity of the second rotor is proportional to the effective piston surface of the associated second cylinder chamber.
Since it may be difficult to make the volumes comply precisely to each other, during floating operation negative pressures may occur in the system. These may have a negative effect on the efficiency and the properties of the hydraulic fluid and lead to cavitation in the system. Therefore, it is advantageous for a reliable operation to provide check valves that permit a flow of fluid from the reservoir to the hydraulic cylinders, as long as the pressure in a hydraulic cylinder chamber drops below a pre-determined pressure.
The hitch is preferably a three-point implement hitch in which the lower arms are not connected over lift struts to a lift shaft, but are connected through a hydraulic cylinder each configured as a lift cylinder in order to raise and lower them. During floating operation the hydraulic cylinders are separated from the pressure supply, that is used for the raising and lowering of the hydraulic cylinders, and connected with flow dividers which synchronize the cylinders.
This hitch or implement interface can be attached to the rear of a tractor, or it can also be used as a front power lift, and is preferable to previous arrangements in which the lower arms of the front power lift are rigidly connected to each other in order to attain synchronization. In this case, however, an inclined position in which the two transverse elements occupy different positions in height, was not possible.
In one embodiment, the hitch includes hydraulic cylinders arranged in the form of a hexapod, such as described in German patent application DE 199 51 840.8, which is incorporated by reference herein. In this case, at least two of the six hydraulic cylinders of the hexapod are connected as described above with a motor/pump unit. Preferably, the two hydraulic cylinders of the hexapod which extend generally in the vertical direction and therefore absorb vertical forces, are connected to one or two motor/pump units.
In order to move the implement to a desired position, the piston end chambers and/or rod end chambers of the hydraulic cylinders are connected via a control valve with a pressure source. The control valve permits a switch between a floating operational mode, in which the motor/pump units are effective, and an operating position in which the hydraulic cylinders can be connected to a hydraulic pressure source. The valve arrangement may be used to control and flow of hydraulic fluid to and from the hydraulic cylinders.